1. Field of the Invention
This invention relates generally to a servo system for controlling the rotational phase of a rotating body, and is directed more particularly to a servo system which employs a digital servo loop to control the rotational phase of the rotary head drum in a VTR or video tape recorder.
2. Description of the Prior Art
The rotational phase of a rotary head drum used in a VTR can be servo controlled either by an analog servo circuit or by a digital servo circuit.
A typical analog phase control servo provides an analog signal, for instance, a DC voltage level, which corresponds to the difference in phase between a reference pulse and a pulse corresponding to the actual phase or rotational position of the rotary recording or reproducing heads. The analog signal is, in turn, used to control the speed or phase of the rotary heads.
A phase compensation circuit is included in the analog servo circuit, to emphasize the lower frequency phase deviations in order to compensate for the relatively slow phase changes caused by DC drift. This circuit normally incorporates an integration circuit, having a relatively long time constant, which is used for forming trapezoidal waves with relatively long slope portions that are sampled by a reference pulse. While the foregoing circuit compensates for relatively slow changes caused by DC drift, such a circuit has a poor transient response. The analog servo circuit is thus insensitive to the relatively fast phase fluctuations that produce jitter and picture instability. However, the analog servo circuits are generally quite simple in construction and inexpensive as compared with typical existing digital phase control servos.
A typical digital phase control servo circuit includes a means to digitize the difference in phase between a reference pulse and a control pulse generated in synchronization with the rotation of the rotary head drum of the VTR. The digitized signal so produced is used to control the speed or phase of the rotary head drum. Such a digital servo is able to respond to slow changes in phase without the necessity of the long-time-constant phase compensator. The transient response is improved substantially as compared with that of an analog servo circuit.
Unfortunately, in a digital phase servo, quantizing errors will occur, because the phase error is quantized to the nearest binary number, and the digital phase servo provides phase correction signals as a voltage stepped in discrete levels. Quantizing errors result when the voltage necessary for phase correction lies between two discrete voltage steps. Such quantizing error results in so-called quantizing noise which, in turn, produced jitter and degrades the reproduced picture quality in the VTR.
In order to suppress jitter caused by quantizing noise, it has been necessary to decrease the size of the stepped voltage levels by increasing the number of bits in the quantized phase correction signal. At the onset of rotation, or at any time that the recording head has scanned an unrecorded portion of tape and beings to scan a recorded portion of tape, the rotational phase of the head is likely to be severly out of phase with the reference signal. In order to bring the rotary heads into correct rotational phase while maintaining the desired reduced voltage level steps, it has been necessary to further increase the number of bits for responding to such large deviation.
For example, if high-speed phase compensation quality similar to that of an analog servo circuit is to be obtained, the bit number of the digitizing means must be on the order of fifteen to twenty bits, which adds considerably to the construction, and cost of the circuit.